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  1. To assess the renal function.
  2. As a routine test in the patient with dialysis.
  3. To assess liver function.
  4. This may be part of the routine test.
  5. In patients has nonspecific symptoms.
  6. Inpatient during the hospital stay.
  7. In patients prior to some drug therapy.
  8. In acutely ill patient admitted in an emergency.


  1. Protein intake will affect BUN. Low protein diet will give low BUN.
  2. High protein diet or nasogastric tubing will increase BUN.
  3. Keep in mind muscle mass which is more in males than females and children.
  4. Overhydrated patient will dilute the BUN and gives a lower value.
  5. The dehydrated patient will concentrate BUN and gives high value.
  6. GI bleeding can cause an increase in BUN level.
  7. Advanced pregnancy may increase the BUN level.
  8. Drugs increasing the BUN level are cephalosporin, indomethacin, gentamicin, polymyxin B, rifampicin, bacitracin, neomycin, tetracycline, thiazide diuretics, and aspirin.
  9. There are drugs which decrease the BUN level are streptomycin and chloramphenicol.


  1. Blood urea molecule: O = C = ( NH2 )2
  2. Blood urea nitrogen is the main waste product of protein metabolism.

  1. Urea forms in the liver with CO2 and is the final product of protein metabolism.

  1. Urea is freely filtered and then partially absorbed by the nephron.
  2. The BUN is used as the index of glomerular function in the production and its excretion of urea.
    1. Urea reabsorption is increased in hypovolemia, so BUN will underestimate Glomerular filtration rate (GFR) and more in hypovolemia.
    2. BUN measures the nitrogen part of the urea.
  3. This BUN or urea excreted through the kidney in the urine.
  4. The measurement of urea nitrogen gives an idea of the ratio between excretion and production of urea.
  5. In the liver amino acids are catabolized and free ammonia is produced.
    1. Ammonia molecules combines to form urea.
    2. This urea through blood goes to kidney and excreted in the urine.
  6. The BUN is directly related to the metabolic function of the liver and excretory function of kidneys.
  7. In chronic renal diseases, the BUN level correlates better than creatinine with the sign and symptoms of the patient.
  8. As the synthesis of BUN depends upon the liver so patients with the severe primary liver disease will have decreased BUN.
  9. In combined liver and renal disease as in hepatorenal syndrome, the BUN may be normal because of poor liver function resulting in decreased formation of urea.
  10. The BUN is less accurate than creatinine for renal diseases.
  11. Breakdown of the proteins and nucleic acid give rise to a non-protein nitrogenous compound in the blood and these are urea, amino acids, urates, ammonia, and creatinine.
  12. High protein diet may increase the BUN and low protein may decrease its level.
  13. Blood urea nitrogen and creatinine ratio also gives the idea about the renal, pre-renal or post renal diseases.

Kidneys dysfunction may show:

  1. The patient may have edema around the eyes, legs, abdomen, and wrist.
  2. There is a history of fatigue, poor appetite, lack of concentration and disturbed sleep.
  3. There may be a flank pain, in the kidneys area.
  4. There may be burning urination, abnormal discharge, and increased frequency.
  5. There is a decrease in the amount of urine.
  6. The urine is bloody or coffee colored and foamy.
  7. There may be hypertension.


Source 1

Source 2

Age Urea nitrogen mg/dL
Cor blood  21 to 40
Premature one week  3 to 25
<1 year 4 to 19
Infant/child 5 to 18
18 to 60 year 6 to 20
60 to 90  8 to 23
>90 years 10 to 31

Source 3

The level above 100 mg/dl is the critical value indicating severe renal dysfunction.

Increased Urea (BUN) Azotemia seen in:

A. Impaired renal function:

Prerenal causes:
  1. congestive heart failure and Myocardial infarction.
  2. Salt and water depletion
  3. Shock
  4. Stress
  5. Acute MI
  6. Hemorrhage into GI tract
  7. Dehydration.
  8. excessive protein catabolism.
  9. Burn.

B. Chronic renal diseases;

Renal causes
  1. Glomerulonephritis.
  2. Pyelonephritis.
  3. Diabetes mellitus with ketoacidosis.
  4. Anabolic steroids use.

C Urinary tract obstruction:

Postrenal causes
  1. Ureteral obstruction from stones, tumors, or congenital abnormality.
  2. Bladder outlet obstruction from prostatic hypertrophy, cancer,
  3. Bladder / urethral congenital abnormality.

Decreased Urea (BUN) seen in:

  1. Liver failure.
  2. Malnutrition, and low protein diet.
  3. Impaired absorption of Celiac disease.
  4. Syndrome of inappropriate antidiuretic hormone secretion.
  5. Pregnancy.
  6. Overhydration.

Effect of drugs and other condition on a BUN:

  1. Some of the drugs may cause a decrease in the BUN like Dextrose infusion, Phenothiazine, and Thymol.
  2. Increased BUN level may be seen in late pregnancy and infancy because of increased use of proteins.

Formula to calculate the Creatinine clearance:

Creatinine clearance =  

Corrected Creatinine clearance =  

 Example if   U = Urine creatinine in mg/dL 

                         V  = urine output in 24 hours (1440 minutes)

                         P = Plasma or serum creatinine in mg/dL

                         A = Body surface area in squamous meter

      Formula  =     

BUN / Creatinine ratio:

  1. The normal BUN / creatinine ratio is around 10:1.
  2. BUN/creatinine ratio in the normal range, in the case of raised BUN and creatinine.  This will suggest intrarenal disease:
    1. Glomerulonephritis.
    2. Tubulointerstitial nephritis.
  3. When the BUN / Creatinine ratio is raised will suggest:
    1. Prerenal azotemia.
    2. Postrenal azotemia.
      1. These conditions may be seen in hypovolemia or hypotension.
  4. BUN/creatinine ratio decreased is very rare and this may be seen in:
    1. protein in diet deficiency.
    2. In severe liver disease. 

Possible References Used

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